Conventionally, as a coating composition including a composition for a paint material, a composition with particles blended therein has been used in order to attain various objects utilizing light diffusibility possessed by the particles. For example, for the purpose of matting surfaces of various molded articles, there has been used a coating composition in which inorganic particles such as silica particles or organic polymer particles (resin particles) are blended. Since a coated film obtained from the above coating composition has a surface on which irregularities are formed and reflection of light is diffused, a surface is matted.
In addition, in an image display device used in a TV set, a personal computer, an electronic notebook, a cellular phone, an amusement apparatus and the like, an optical sheet for diffusing transmitted light or reflected light is used as a light diffusing sheet or an antiglare sheet. Also in this optical sheet, in order to diffuse the transmitted light or the reflected light, a coated film is formed on a surface thereof. As a coating composition for forming the coated film on the surface, there has been used a composition in which inorganic particles or resin particles are blended.
For example, in the light diffusing sheet for diffusing the transmitted light, as a coating composition, there has been used a composition with particles of inorganic substances such as calcium carbonate and silica, or organic polymers such as polystyrene and silicone blended therein.
However, the inorganic particles have a problem of low dispersion stability in a binder solution, in particular, a binder solution using a transparent binder such as a polymethyl methacrylate resin as a binder resin.
In addition, in a coating composition with organic polymer particles blended therein, dispersion stability is better, but there is a problem that sufficient light diffusibility is not obtained.
On the other hand, in optical components such as an illumination apparatus cover, a lens, a light guiding plate, a video disc, a screen for a projection television, and various molded articles such as a cosmetic container, a front plate of a vending machine, a signboard, a merchandise display and a table container, in order to enhance a merchandise value such as their aesthetic property, improvement of light diffusibility has been challenged by molding the above articles using a raw material in which light diffusible particles are blended in a resin. In these molded articles, various resins, in particular, thermoplastic resins such as a polycarbonate resin, a polystyrene resin and a polyacrylate resin are used as a base material.
Conventionally, as the light diffusible particle used in the molded articles, there have been well-known inorganic particles such as glass, calcium carbonate and silica, or resin particles such as a polyacrylate resin and a polystyrene resin.
However, these molded articles have a problem that transparency is degraded due to blend of the light diffusible particles and, also, improvement in light diffusibility is not sufficient.
In response to these problems, Japanese Patent No. 3,040,705 proposes a polymer particle containing smectite, and Japanese Unexamined Patent Publication No. Hei 10(1998)-265580 proposes a composite particle in which silica particles of 500 nm or less are dispersed in resin particles.
However, even a composition using one of the above particles as particles to be blended can not still provide a coated article or a molded article having sufficient light diffusibility.
Additionally, in addition to those described in the above publications, a composite particle in which a surface of a polymer particle is covered with other polymer or an inorganic material has been also known.
Among these, the latter composite particle in which the surface of the polymer particle is covered with the inorganic material has an advantage that a refractive index at an interface between the polymer particle and the inorganic material is made greater than a refractive index at an interface between the polymers of the former.
A composite particle in which a surface of polymer particle is covered with an inorganic material has been reported, for example, in Japanese Unexamined Patent Publication No. Hei 5(1993)-170924. This publication describes a composite particle in which a substance smaller than a particle of a thermoplastic substance and excellent in heat resistance is immobilized on the particle of a thermoplastic substance. The composite particle is obtained by a mechanical shear force by raising a temperature of the particle of the thermoplastic substance to a softening temperature or more, and stirring a substance excellent in heat resistance and the particle of the thermoplastic substance.
The composite particle obtained by the method of Japanese Unexamined Patent Publication No. Hei 5(1993)-170924 has a structure that the surface of the particle of the thermoplastic substance is approximately uniformly covered with the substance excellent in heat resistance, has slightly improved in light diffusibility and reflectivity rather than the particle of the thermoplastic substance as a raw material; however, further improvement is demanded.
In addition, in the above method, since a mechanical shear force is applied to the particle of the thermoplastic substance, it is required that the particles of the thermoplastic substance endure this shear force, and there is a problem that a usable substance is limited.
Furthermore, it is required that the particle of the thermoplastic substance is prepared in advance before subjected to the method, and further omission of a step is desired from a viewpoint of saving of a manufacturing cost.